1. Field of the Invention
This invention relates to a direct conversion receiver which can be used in a mobile communication apparatus such as a PHS (Personal Handy Phone) telephone set, a portable telephone set, or a pager.
2. Description of the Related Art
Japanese published unexamined patent application 6-54005 discloses a direct conversion receiver having an FSK (frequency shift keying) demodulator. In the direct conversion receiver of Japanese application 6-54005, a received radio-frequency (RF) FSK signal carrying a digital information signal is fed to first and second mixers. The first mixer down-converts the RF FSK signal in response to a first local oscillator signal. The second mixer down-converts the RF FSK signal in response to a second local oscillator signal having a quadrature relation with the first local oscillator signal. The first local oscillator signal is outputted from a voltage controlled oscillator which serves as a local oscillator. A 90.degree. phase shifter converts the first local oscillator signal into the second local oscillator signal. The output signal of the first mixer is processed by a first low pass filter, being converted thereby into a baseband I (in-phase) signal. The output signal of the second mixer is processed by a second low pass filter, being converted thereby into a baseband Q (quadrature) signal. A demodulation circuit recovers the digital information signal from the baseband I signal and the baseband Q signal. The demodulation circuit outputs the recovered digital information signal.
In the direct conversion receiver of Japanese application 6-54005, one of the baseband I signal or the baseband Q signal is converted into a signal voltage by a frequency-to-voltage converter. The signal voltage is compared with a reference voltage by a voltage comparator. An in-phase/opposite phase discrimination circuit compares the phase of the output signal of the voltage comparator and the phase of the output signal of the demodulation circuit to decide whether the phases are equal or opposite to each other. When the phases are equal to each other, the in-phase/opposite phase discrimination circuit outputs a voltage-down signal to a charge pump. When the phases are opposite to each other, the in-phase/opposite phase discrimination circuit outputs a voltage-up signal to the charge pump. The charge pump generates a voltage signal in response to the voltage-up signal and the voltage-down signal outputted from the in-phase/opposite phase discrimination circuit. The voltage signal is transmitted from the charge pump to a control terminal of the voltage controlled oscillator via a low pass filter. Thus, the frequency of the first local oscillator signal and also the frequency of the second local oscillator signal are controlled in response to the voltage signal generated by the charge pump. This frequency control is designed to provide automatic frequency control (AFC) which enables the frequency of the first and second local oscillator signals to automatically follow the carrier frequency (the center frequency) of the received RF FSK signal. The selectivity of the direct conversion receiver of Japanese application 6-54005 is determined by the cutoff frequency of the first and second low pass filters which follow the first and second mixers. The cutoff frequency of the first and second low pass filters is fixed. Therefore, in the direct conversion receiver of Japanese application 6-54005, it is difficult to automatically adjust the selectivity in response to signal receiving conditions.
As previously explained, the direct conversion receiver of Japanese application 6-54005 has the AFC function of correcting an error between the frequency of the first and second local oscillator signals and the carrier frequency (the center frequency) of a received RF FSK signal. The AFC can not correct a frequency error having a magnitude corresponding to greater than a passband provided by the receiver selectivity. The passband is fixed since the cutoff frequency of the first and second low pass filters and also the receiver selectively are fixed. Thus, in the direct conversion receiver of Japanese application 6-54005, it is difficult to automatically adjust the greater limit of a frequency error which can be corrected by the AFC.